Male and Female Morphology Is Environmentally Determined
AMHERST, Mass.-Differences between males and females may be caused by ecological factors-if you're a purple-throated carib hummingbird. Ethan Temeles, assistant professor of biology, and three of his students from Amherst College report their findings in the July 21 issue of Science.
Temeles, Irvin Pan '99, Jill Brennan '01 and Jedidiah Horwitt '01 traveled to the Caribbean in 1999 to look for evidence of ecological causation of sexual dimorphism in bill morphology of Eulampis jugularis. In other words, they wanted to know: "Can the shape of the flower make the shape of the female hummingbird's beak different from the male's?"
The flower and the bill do seem to have co-evolved in an unusual manner, according to Temeles's research. The male birds, which feed predominantly on a red-bracted variety of heliconia, have relatively short and straight beaks. The females feed on a green-bracted heliconia, and their beaks are longer and more curved. The island of St. Lucia offered a rare opportunity for the researchers, because the hummingbirds there feed almost exclusively on heliconia, which is in turn pollinated exclusively by them. This allowed the team to carefully count and measure which birds fed at which plants.
Temeles says the idea that the ecology can affect sexual dimorphism "comes straight from Darwin," who suggested three possible reasons that animals of the same species but different sex and should evolve differently. Sexual selection is the best known mechanism; the male peacock's resplendent tail helps him attract a mate. The hypothesis of "fecundity selection," which Darwin also suggested, notes simply that, for example, a larger female spider will be able to carry more eggs than a smaller female.
Darwin also believed that ecology could influence the differentiation of the sexes. But with the exception of some mosquitoes-the female's mouths are adapted to suck blood, the male's nectar-no clear example of ecological causation of sexual dimorphism had been found until Temeles and his students researched the purple-throated carib.
"So Darwin was right about sexual differences in hummingbirds being caused by their ecology," Temeles says. "Now what other examples of ecological causation of sexual dimorphism are out there?"
Temeles's home page is at http://www.amherst.edu/~ejtemele/